At one time, this may have been a question that underachieving high school students asked each other as they made their way to their evening shift at Burger King. However, as the frequency and legitimacy of marijuana usage increases, the question may start popping up in professional workplaces, too.

In Canada, the projected number of licensed users of medical marijuana is expected to hit 433,638 by 2024. Once legalization takes place, it could become commonplace to see people lighting a joint on their way to work.

So how will companies handle their employees' marijuana usage and determine whether or not it has a negative impact on their work? According to George Waggott, a labour-relations and employment-law partner at McMillan LLP in Toronto, this will be tricky. For starters, it's challenging to prove an employee's usage, since unlike with alcohol, a blood test or Breathalyzer test cannot easily measure it. Even when employees acknowledge they are using marijuana, the medical community remains conflicted over what constitutes impairment by THC (tetrahydrocannabinol), the psychoactive ingredient in marijuana.

"Even in cases where the employee either admits to being high or is found to be, likely based only on observations as opposed to testing, there will still potentially be a dispute about whether or not this is grounds for termination," Mr. Waggott said.

Naturally, the greatest concern for employers revolves around workplace safety. However, in the work force at large, that constitutes a fraction of jobs.

So, if you aren't driving a forklift all day, Mr. Waggott said that the acceptance of usage at work comes down to company policy. While employees are not normally required to disclose medical conditions, they can be bound to disclose if there is a specific company policy. In some cases, the rights of the employer may clash with the employee's right to medically use the drug.

"As with any other workplace accommodation issue, the key starting point for the employer is to get proper and current information about any restrictions which the employee has. In a large number of cases, the employee does not necessarily even need to use the drug during working time and indeed they will not be impaired. So the only accommodation in that scenario is to recognize that if you ever tested them, there will likely be a positive result," Mr. Waggott said.

If the employee must partake of medical marijuana during working hours, the obligation of the employer may be to provide adequate time off – either paid or unpaid – as they would for a medical appointment.

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For the most part, Mr. Waggott said the majority of employers he has worked with have shown great deference to the rights of the users. The tricky part comes down to how much or how little the use of medical marijuana will negatively affect the employee's work.

If employees use the drug responsibly, there really shouldn't be any issue, argued Neil Closner, chief executive officer of Toronto-based MedReleaf Corp., the largest licensed producer of medical cannabis in Canada. Additionally, he said companies such as his own offer a variety of cannabis products that contain almost no THC.

"MedReleaf has a small number of staff who have legitimate need for cannabis and we happily accommodate them," Mr. Closner said, adding that in a few months, his company will be introducing health-insurance coverage for medical cannabis as part of its employee-benefit plan.

The drug, he argued, is here to stay, so companies and employers may as well start getting used to it.

Indeed, the popularization of the drug as Canada moves toward legalization may even make its use in the workplace no big deal.

"I may be in a minority on this, but I think legalization of marijuana will not impact workplace use. In fact, by removing the stigma of illegality, we may even see a more responsible approach," Mr. Waggott said.

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"People know that being drunk at work is bad. They arguably did not know that 30 years ago, but times have changed, and so have the accepted societal norms. Legalizing marijuana is likely going to foster something similar – just because you can do it when you want does not mean you should always do it," he said.

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